198 A. LWOFF 



Sometimes, however, the infection is "abortive." Phage proteins and 

 nucleic acid are synthesized, but are not organized into infectious particles. 

 The disease has been initiated by an infective viral particle and it is character- 

 ized by the production of viral material. It is a viral disease, but it is not 

 infectious sensu stricto. When phage is produced from prophage, it is the 

 reverse: the disease has been initiated by a noninfectious particle and ends 

 with infectious viral particles. The disease is again Yiial, but again it is not a 

 "true" infectious disease. 



Some lysogenic bacteria called defective perpetuate an abnormal prophage. 

 When induced, these lysogenic bacteria synthesize phage material and die, 

 but do not hberate phage particles. The disease is not infectious. To decide 

 that it is or not viral is a matter of taste. 



Finally, some bacteria, called bacteriocinogenic, perpetuate a potentially 

 lethal gene. When this gene is expressed, the bacteria start synthesizing 

 bacteriocins and die. The disease is a lethal synthesis which is neither viral 

 nor infectious. The production of bacteriocin is a bacterial disease 

 controlled by a potentially lethal gene. 



Prophage is also, in a way, a potential lethal gene, for the lysogenic 

 bacterium dies when prophage expresses its potentialities. As already seen, 

 the originahty of the disease is that it ends with organized infectious 

 particles which are viruses. And the originality of lysogenic bacteria is the 

 perpetuation of a structure which behaves as a bacterial gene and which is the 

 genetic material of a virus. 



It is therefore useful to remember that viral diseases, considered at the 

 level of individual cells, are sometimes not infectious, that a virus is not 

 always pathogenic and not always infectious. The attributes of viruses — 

 reproduction, infectivity, pathogenicity — are never expressed at the same 

 time in the same structure. 



These essential features are present 'potentially in the genetic material, 

 which is the only essential, nondispensable part of a virus. It contains the 

 information necessary for the synthesis of viral constituents and for the 

 morphogenesis of organized infectious particles. Many virologists are 

 inclined to think that the genetic material preceded the virus in phylogeny. 



The virus thus occupies a privileged position at the crossroads of normal and 

 pathological heredity, at the crossroads of diseases of the metabolism and 

 infectious diseases. 



VIII. Eemarks on the Pathogenicity of Viruses 



Infectivity and pathogenicity have been considered as indispensable 

 characters for identifying an infectious agent as a virus. So far as infectivity 

 is concerned, this seems justified, but the inclusion of pathogenicity, even of a 

 potential pathogenicity, as a necessary feature of a virus may be questioned. 



